Most current and anticipated optical fiber communication systems use silica-based optical fiber, typically single mode fiber with a Ge-doped core.
It is known that exposure of such fiber to H.sub.2 can result in an increase in the attenuation of signal radiation in the fiber. However, under typical operating conditions such H.sub.2 -induced loss is not a significant problem for standard single mode Ge-doped silica-based fibers. See, for instance, A. Tomita et al., Electronics Letters, Vol. 21, p. 71 (1985), which predicts loss increases at 1.3 and 1.55 .mu.m of less than 0.01 dB/km after 20 years. See also P. J. Lemaire et al., Conference Proceedings, 10th European Conference on Optical Communications, September 1984, Stuttgart, which discloses that "conventional" single mode fibers containing Ge, P, and/or F are very resistant to OH formation upon exposure of the fiber to hydrogen at elevated temperatures, but that " . . . an alumina doped fiber reacted quickly to form Al--OH".
It is also known that fiber can be provided with a (exemplarily carbonaceous) coating that is substantially impermeable to H.sub.2 and H.sub.2 O. See, for instance, U.S. Pat. No. 5,000,541. Such "hermetic" fiber can be advantageously used in applications such as oil well logging or undersea systems. The '541 patent also discloses use of "getter" sites to bond hydrogen that is diffusing into the cladding material of the fiber, such that the hydrogen does not reach the optically active region (consisting of the core and, possibly, a minor part of the cladding that is immediately adjacent to the core) of the fiber. See also P. J. Lemaire et al., Materials Research Society Symposium Proceedings, Vol. 172, p. 85 (1990), which inter alia discloses on p. 96 that " . . . the reaction of H.sub.2 in optically inactive portions of a hermetic fiber can be advantageously used to scavenge trace amounts of hydrogen that might be present, leading to further improvements in fiber reliability".
P. J. Lemaire et al., Technical Digest-Symposium on Optical Fiber Measurements, National Institute of Science and Technology, Boulder, Colo., September 1990, Special Publication No. 792, inter alia disclose a model for the diffusion of hydrogen in hermetic fiber with reactive (gettering) sites and show experimental results for conventional fiber.
U.S. Pat. No. 5,059,229 addresses the "transient hydrogen sensitive attenuation phenomenon" and discloses drawing fiber from the preform while a H.sub.2 -containing gas is present in the draw furnace.
A. Oyobe et al., Technical Digest, Conference on Optical Fiber Communication, San Diego, Calif., February 1992, disclose a tightly coiled hermetic erbium-doped fiber (Er-doped Ge-silica center core, Ge-silica side core, F-doped silica cladding). The hermetic carbon coating was provided to prevent mechanical fatigue of the 200 m long fiber. The coil was designed for use in a compact optical fiber amplifier.
It is known to co-dope Er-doped amplifier fibers with Al. It is believed, inter alia, that the presence of alumina in the central core region makes it possible to attain higher Er-levels in that region than would be attainable in an Al-free fiber. See, for instance, U.S. Pat. No. 5,058,976.